Im not sure if that will add that much value for tesla, sure they can bring down the import tax by some amount.
But the chinese customer is a very special kind of customer, one which likes to spend 2x the money if the product is NOT made in China
http://www.reuters.com/article/2013/12/16/us-china-luxury-idUSBRE9BF0QD20131216
Also the chinese customer is this price has to much "pride" to spent not enough money in a Car.
The Model S is already much cheaper in China than any of it competitors, and the worldwide demand for the Model X and Model S stay bigger then the supply for years to come.
I also read that a lot of Chinese prefer to import a Car Model from Europe, rather the buying the Chinese build which is much cheaper due to import tax. (Its exactly the same Car/Model etc.)
For the GenIII although that would be a good move.